The densification of propellants for vehicles has been the subject of attention because of the advantages of densification. Densification reduces the volume of a given mass of propellant, and therefore reduces the volume of the tank which is required to hold a given mass of propellant. This in itself is an advantage, in that it decreases the weight of the tank, and thereby increases the range of the vehicle, at least in principle. When the densification is accomplished by cooling, a further, less obvious advantage arises, namely that the vapor pressure of the propellant is lowered. The lowering of the vapor pressure reduces the pressure which the tank must withstand, so that the tank structure may be made thinner and therefore less massive, which further reduces the weight of the tank over and above the reduction occasioned by the densification itself.
FIG. 1 is a simplified representation of a prior-art arrangement for cooling a cryogenic propellant. In FIG. 1, a vehicle 10 may be, for example, a spacecraft. Spacecraft 10 includes at least one propellant tank, which holds a monopropellant if appropriate, or if two tanks are available on vehicle 10, one will contain fuel, and the other oxidizer, for use in a propulsion engine (not illustrated). As illustrated in FIG. 1, a circulating pump external to vehicle 10 pumps propellant from tank 12, by way of a pipe or channel 16, through a heat exchanger 18 located within a tank 20, and back to the tank 12 by way of a path 22. Tank 20 is filled, at least to the level of the top of the heat exchanger 18, with a cryogenic liquid 24, which is preferably one of the propellants, already available on the site.
In order to densify the propellant in the tank 12 of vehicle 10, the pressure of the gaseous phase of cryogenic liquid 24 is reduced by a compressor bank which is used as a cryogenic pressure reduction pump 26 connected to tank 20 at a port 21. The compressed gas is vented by a vent 28. The reduction in the pressure at the surface of cryogenic liquid 24 within tank 20 promotes vaporization of the liquid component, which causes heat to be removed from the liquid. The volume of cryogenic liquid 24 within tank 20 is maintained by a connection at a port 31 to a source of cryogenic liquid, illustrated as a vented tank 30.
It will be appreciated that a vacuum pump such as that represented by block 26 of FIG. 1 must operate continuously for many hours in order to accomplish densification of sufficient propellant for a large vehicle. A small-scale test of a system equivalent to that of FIG. 1 was performed. The manufacturer of the compressor bank which is used as a cryogenic pressure reduction pump indicates that fabrication of a single pump suitable for propellant densification for a full-size vehicle is beyond the state of the art, and as many as forty paralleled maximum-size pumps would be required to handle the densification task for such a vehicle. The reliability of such maximum-size cryogenic pumps might result in some failures among the paralleled pumps during the densification process associated with a launch.
Other possible approaches which have been suggested include the use of commercial steam ejectors in conjunction with a steam generating plant. A steam ejector is basically a Bernoulli-type vacuum generator in which pressurized steam is ejected from a nozzle and generates a partial vacuum. However, the availability of water for generation of steam at the launch site was deemed inadequate to accomplish the desired result, and burning of enough diesel fuel to generate the steam, even if sufficient water were available, was deemed to be environmentally unsound.
Yet another possibility which was considered was to combust hydrogen and oxygen, already available at the launch site, thereby generating high-energy steam for use in a steam ejector. This "Lox/Hydrogen Torch," however, is basically a rocket engine which is required to burn at some 6000.degree. F. for several hours. Such a rocket engine has never been attempted, and would require a major investment to design, build and test, assuming that such a structure could be made and operated for the desired purpose.